The present invention relates to marine propulsion, more particularly to methods, apparatuses and systems for effectuating marine propulsion which implements one or more pumpjet-type propulsors.
Open propellers on outboard motors represent a hazardous condition. This potential for personal injury due to open propellers has provided impetus for the development of pumpjet-type propulsors which enclose the rotating elements of the thruster. The application of pumpjets to outboard motors is a relatively recent development.
An outboard motor-mounted pumpjet (alternatively spelled "pump-jet" or "pump jet") propulsor is capable of performance which is comparable to, or even exceeds, that of a suitably designed open propeller. However, one of the drawbacks associated with pumpjet implementation is the increase in wetted surface area due to the presence of the shroud (duct) around the impeller; this increased wetted surface area results in increased lower unit drag. This drag phenomenon limits maximally attainable craft speed and otherwise impairs craft performance.
Various approaches have been proposed for alleviating the drag problem attendant pumpjet propulsion. Such approaches have generally involved complexities which may be undesirable for practical or economic reasons. For instance, utilization of special ducting has been conceived for purposes of directing air or engine exhaust to the shroud area or nozzle area of the pumpjet.
In the early 1990's, the U.S. Navy's Naval Surface Warfare Center, Carderock Division (NSWCCD), was tasked (in a private intragovernmental context) by the U.S. Marine Corps (USMC) to develop an improved pumpjet. The 35 SHP outboard pumpjet, which was commercially acquired and used by the USMC at that time, performed well but was characterized by certain performance penalties when compared to an open propeller used on the same outboard motor. This endeavor by the U.S. Navy to improve pumpjet performance has led to the present invention.